Fuel injectors (nozzles) are important components of gas turbines as well as other gas combustion engines. Because the fuel injector is the source of the fuel, the fuel injector can provide significant play in the role of engine performance.
Because the fuel injector extends into the engine case and particularly between the compressor and the combustion chamber in a gas turbine, typically, a fuel injector includes an external support/stem through which an internal fuel tube extends to support and protect the fuel tube. The fuel tube will be connected to an atomizer or other tip to improve the delivery state of the fuel into the combustion chamber so that it will more fully mix with air in the combustion chamber.
During operation, the support/stem is surrounded by high-temperature and high-pressure compressor air within the compressor discharger cavity where the air exits the compressor. However, it is desirable to deliver the fuel at a much lower temperature than the compressor air and therefore to prevent heat transfer from the compressor air to the support system and ultimately the fuel. Particularly, because if too much heat is transferred to the fuel, the fuel can begin to coke, thereby ruining or reducing the quality of the fuel. Additionally, coke depositions may occur that further inhibit the efficiency of the fuel injector. There have, therefore, been attempts to reduce the amount of heat that can be transferred from the high-temperature compressor air to fuel passing through the fuel injector.
Unfortunately, the support/stem is typically a solid cast piece that can allow for significant heat transfer. Attempts to reduce heat transfer to the fuel have included surrounding the stem/support with a heat shield. Unfortunately, past attempts to include a heat shield have directly connected the heat shield to the support/stem by soldering or brazing. For example, this type of connection can be seen in U.S. Pat. No. 6,149,075 issued Nov. 21, 2000 to Moertle et al where a butt weld serves an end of the heat shield to an overhang flange of the support system.
A first problem with this arrangement is the attachment locations create a heat transfer path providing heat flux short circuiting from the air flow to the injector, defeating the thermal protection provided by the heat shield. This is amplified by the fact that the attachment locations or connection between the heat shield and support/stem is typically positioned within the combustion chamber further promoting heat transfer between the heat shield and the support/stem.
Additionally, as this junction promotes localized heat transfer at the point of the junction, thermal gradients are also created at the site of the junction creating thermal stresses. These thermal stresses are further compounded by the fact that these junctions are typically butt or lap type weld joints which are inherently less reliable.
Finally, as the heat shield is typically connected at or proximate to opposite ends of the support/stem, the thermal growth differential between the heat shield and the underlying cooler support/stem creates additional stress fights within the fuel injector, and particularly the heat shield and the support/stem
The present invention relates to improvements over the current state of the art in fuel injectors.